Recently, white light emitting diodes (referred to as white LED) are introduced into the market. Since the white LED has the power-saving benefits, the white LED will gradually replace the conventional fluorescent tube.
Generally, the current white LED uses a phosphor to mix yellow light with blue light to produce white light. For example, when the blue light with a peak wavelength in the range between 450 and 470 nm is absorbed by a yellow phosphor YAG:Ce, a light with a peak wavelength in the range between 550 and 560 nm and a radiation wavelength in the range between 450 and 700 nm is generated. Consequently, the blue light and the yellow light are mixed to produce the white light.
FIG. 1 schematically illustrates the spectrum of a white LED that is produced by a conventional blue LED and a YAG phosphor. As shown in FIG. 1, the high-brightness blue LED with a peak wavelength in the range between 445 and 450 nm is used to excite the YAG phosphor. Since the blue color and the yellow color are two complementary colors, the blue light and the yellow light may be mixed to produce the white color. In other words, the conventional blue LED and the YAG phosphor may be combined as a white LED.
Besides, a UV LED with a peak wavelength in the range between 350 and 430 nm may be used to excite an RGB phosphor. Consequently, the UV light and the RGB light are mixed to produce the white color.